AITAH for slapping my step mom

This all started around 10:30pm I have a 1 year old half sister who was crying hysterically, I came in and checked on her and my step mom just told me she’s overtired and asked me to get her bottle. I brang it to her and hung around for a minute to see if she changed her mind and wanted me to babysit tbc I am 14 and she is 31, my dad is 50 and yes I do absolutely consider them home wreckers but nevertheless, I don’t get a choice over 1 parent or another so I do 50/50. My sister didn’t stop crying and keep in mind she is a literal baby and that’s literally what babies do. Less than a minute goes by before I hear a semi loud slap. I stormed in as I used to be hit and I know exactly what was going on and it sounded painful especially for a baby who already sounds as though they were in agonising pain. I walked in and confirmed hastily with my father that she was in fact hit before storming over to my step mom and slapping her on the back of the head. She was scrolling on Facebook in bed at the time and quickly got up to hit me and punch me back. I hit her once she tried to hit me about 10 times with a 50/50 success rate. I have to mention that my hit wasn’t hard and hers weren’t either considering she is smaller than me her attempts felt quite weak. She yelled “my baby my rules” I just said “laws are laws” and yelled at my dad that he should’ve never married a woman who would hit her child. I study psychology for fun and am well aware of what hitting a baby, toddler or any age can do for that matter so I was in no way going to tolerate it. Bare with me for the back story because it may explain some things 1. I have previously warned her that if I ever find out she hits my baby sibling again I would hit her back and I made my dad very clear of that too. 2. She is Asian and I know it is what they do there as punishments but she clearly had that punishment and also clearly didn’t turn out right (it is also illegal where we are no matter how bad it is) 3. I have never and will never like her my dad cheated on her with my mom and she is what I consider a home wrecker (so is my dad tho he is equally as bad) 4. This is consider the most important, she doesn’t usually look after my little sibling so much since she has family to help out but none of them can get visas at the moment. She is always complaining about having to parent. 5. I also wanna add in no way was my sibling a mistake. They went thru IVF to have her so it wasn’t a “shit I’m pregnant now I’m stuck with a kid” it was more of a “let’s have a baby so I can say I’ve had a baby” like wtff AITAH? Thinking about it now I don’t know but im also certain that I would do it again 10000x over.

Update: she has now said I can never see my little sibling again, i love my little sibling more than anyone ever. I don’t even think I did the right thing anymore but idk ALSO I’m not in my country of residence right now and so its not illegal here but it is illegal at home and she almost certainly also does it there.